Mobile Communication Systems

Part II- Cellular Concept

Professor Z Ghassemlooy
Faculty of Engineering and Environment
University of Northumbria
U.K.

http://soe.northumbria.ac.uk/ocr/
Prof. Z Ghassemlooy

Content
Introduction
Cell shapes and clusters
Frequency reuse:
Distance

Efficiency

Cluster size
How to find the nearest co-channel neighbours
Channel assignment strategy:
Capacity
Handoff

Interference:
Signal-to-noise ratio
Prof. Z Ghassemlooy

Cellular - Introduction
Solves the problem of Spectral congestion and user
capacity by means of frequency reuse
Offers high capacity in a limited spectrum allocation
Offers system level approach, using low power
transmitters instead of a single not interfere with the nearest
location, high power transmitter (large cell) to cover larger
area.
A portion of the total channels available is allocated to
each base station.
Neighbouring base stations are assigned different groups
channels, in order to minimise interference.
Prof. Z Ghassemlooy

Cell Shapes

a = 2R2

a = 33/2 R2/16

Not suitable, (different distance from the cells

Centre to different point in the perimeter)

Ideal shape, but

has dead zones

Prof. Z Ghassemlooy

Cell Shapes Hexagonal

Reasons:
The highest-degree of regular polygons that can tile a plane .
Approximate the circular contours of equal received signal strength when
the propagation is isotropic in the horizontal plane.
Only small difference from the centre to other point in the perimeter
Hexagonal cells are widely used to understand
and evaluate system concepts. Is the basic
geographic unit of a cellular system

R : Distance from the centre to

any vertex of the hexagon

BS

Real Cell Shape:

Actual cell shape
System planning, terrain and other effects result in cells that are far less
regular, even for elevated base station antennas.
Base stations location is strongly influenced by the practical problem of
finding acceptable sites and may not follow the regular hexagonal grid.
Prof. Z Ghassemlooy

Cell Size
Macro cell: 10km
Micro cell: 1 km Shopping centres, airports etc.
Pico cells: 4 100 m Inside building
Small cells more bandwidth more users
more base station complex networks
leading to more interference and more hand
overs.

Prof. Z Ghassemlooy

Mobile Communs. - Cellular Spectrum

Phone Transmit
824 825

835

845

30kHz

30kHz

1 MHz
33 chan

849

B band

B band
10 MHz
333 channels

A band

A band

A band
10 MHz
333 channels

846.5

1.5 MHz 2.5 MHz

50 chan 83 chan
20 MHz Guard

Base Transmit
869 870

890
B band
10 MHz
333 channels

30kHz

30kHz

B band

A band
10 MHz
333 channels

891.5 894
A band

A band

1 MHz
33 chan

880

1.5 MHz 2.5 MHz

50 chan 83 chan
Prof. Z Ghassemlooy

Cell Cluster
A cluster is a group of cells
No channels are reused within a cluster
Cell
BS2
BS7

BS3
BS1

BS6

BS4
BS5

A 7 cells cluster
Power distribution
Prof. Z Ghassemlooy

1
2
3
4
5
6
7

Frequency
(MHz)
900
900.3
900.6
900.9
901.2
9001.5
9001.8

Frequency Reuse - Concept

Adjacent cells are assigned different frequencies to

avoid interference or crosstalk

10 to 50 frequencies assigned to each cell

The coverage area of cells is called the footprint and is

limited by a boundary so that the same group of
channels can be used in cells that are far enough apart

The essential idea of cellular radio is to transmit at

power levels sufficiently low so as to not interfere with
the nearest location at which the same channel is
reused.
Prof. Z Ghassemlooy

Frequency Reuse

contd.

BS2
BS3

BS7

U2

BS6
BS2

BS4
BS5

BS3

BS7

BS2
BS7

BS1
BS6

BS4
BS5

Cells with the same

number have the
same set of
frequencies

BS1

BS3

U1

Ui: Frequency re-use

vector

BS1
BS6

BS4
BS5

Prof. Z Ghassemlooy

Frequency Reuse Distance

BS2
BS3

BS7
R

BS1
BS6

BS4
BS5

v1

BS3

BS7

v2

/6

BS2

BS1
BS6

BS4

Dnc

BS5

The displacements between any two cells can be expressed as a

linear combination of the two basis vectors v1 and v2 having an angle
of 60. Then |v1| and |v2| = (3)0.5R.
Or, the centre-to-centre distance between two neighbouring cells is

Dnc 2 R cos ( / 6) or 3R
Prof. Z Ghassemlooy

Frequency Reuse Distance

contd.

Cell area
BS2
BS3

BS7

Dcc
BS4

BS5

BS3

BS7

BS1
BS6

/6

BS2

a = |v1 v2|
= 3R2 sin (30)

BS1
BS6

BS4

3 3 2
a
R
2

BS5 R

The centre-to-centre distance between any two co-channel cells is

Dcc i 2 j 2 ij ( 3R )
Where i = j = 0, 1, 2 etc. represent the centre of a cell (reference). For adjoining
cells, either i or j can change by 1, but not both.
Prof. Z Ghassemlooy

Frequency Reuse Distance

contd.

The greater the reuse distance, the lower the

probability of interference. Likewise, the lower the
power levels used in cells sharing a common channel,
the lower the probability of interference.
Thus, a combination of power control and frequency
planning is used in cellular systems to prevent
interference.

Prof. Z Ghassemlooy

Cluster Size
Area of a region can be expressed by
A = Dcc2 sin 60

The number of cells per cluster within an area of radius Dcc

(i.e in reuse pattern) is:

U1 U 2

Dcc
N

V1 V2
R
1
3

Also N= A/a
Frequency reuse factor = 1/N
Area of the cluster

2
cc

D
A 3
2
Prof. Z Ghassemlooy

Locating Co-Channel Cells

V

To find the nearest cochannel neighbours one

must do the followings:

BS2
BS3

BS7
BS1
BS6
BS2

BS4
BS5

BS3

BS7

BS4
BS5

U
BS3

BS7

BS1
BS6

BS2

1/3

BS1
BS6

BS4
BS5

Prof. Z Ghassemlooy

1. move i cells in the

U direction
2. turn 60o counterclockwise and move
j cells in the V
see Fig. N = 7,
i = 2 and j = 1

Data
Co-channel reuse ratio Q = Dcc / R = (3N)
i

Q=D/R

Transmissi
on quality

Traffic
capacity

1
1
2
2
3
2

0
1
0
1
0
2

1
3*
4*+
7*=
9*
12*+
21*=

1.73
3
3.46
4.58
5.2
6

Lowest

Highest

Highest

Lowest

* Most common, + Digital network,

= Analogue network

Prof. Z Ghassemlooy

Frequency Reuse efficiency

No. of available user channels in real system
fr
No. of available user channels in ideal system
Note: In ideal system there are no co-channel interference

Frequency reuse factor = 1/N

N is the number of channels

Prof. Z Ghassemlooy

Channel Assignment Strategies

The choice of channel assignment strategies impacts
the performance particularly as to how calls are
managed when a mobile user is handed off from one
cell to another.
There are basically two strategies:

Fixed

Dynamic

Prof. Z Ghassemlooy

Channel Assig. Strat. - Fixed

Each cell is allocated a predetermined set of voice channels
irrespective of the number of customers in that cell. This
results in traffic congestion and some calls being lost when
traffic gets heavy
Call attempted within the cell can only be served by the
unused channels in that particular cell
Call is Blocked if channels are occupied
If all the channels are occupied cell may be allowed to use
channels from a neighbouring cell
Used in TDMA/FDMA cellular radio systems
Prof. Z Ghassemlooy

Channel Assig. Strat. - Dynamic

Channels are not allocated to different cells permanently.
Is ideal for bursty traffic
Each time a call request is being made, the serving BS
request a channel from MSC.
MSC allocate a channel by using an algorithm that
takes into account:
- the likelihood of future blocking within the cell
- the frequency reuse of the candidate channels
- the reuse distance of the channels
- cost functions

MSC requires to collect real time data on:

- channel occupancy and traffic distribution
- radio signal strength of the channels on a continuous basis
Prof. Z Ghassemlooy

Channel Assig. Strat. - Dynamic

Since a cell is allocated a group of frequency carries
(e.g. f1-f7) for each user, then
Bandwidth of that cell Bce = a range from carrier
frequencies
Adopted in GSM, DCS and other systems

Prof. Z Ghassemlooy

Channel Capacity
Cluster with size N = 7
BS2

k = Number of channels / cell

BS3

BS7
BS1
BS6
BS2

BS4
BS5

BS3

BS7

BS2

BS4
BS5

BS3

BS7

BS1
BS6

No. of cluster

BS1
BS6

BS4
BS5

Prof. Z Ghassemlooy

Duplex frequency
bandwidth / channel
Total duplex
channels available
for reuse: S = kNB

Channel Capacity

E.g. for GSM:

Normally 25MHz/200kHz/channel = 125 channels /cluster
For N = 7, k = 17 18 channel/cell
And for M = 3,
C = 3 x 125 = 375 channel
Or for
S = 8, N = 9, and B = 2 x 200 kHz = 0.4 MHz.
Thus k = 2.2 channels. Cell-1.MHz-1
For analogue systems
k = 1.9 channels. Cell-1.MHz-1
Prof. Z Ghassemlooy

Cellular Network
Network and Switching
Subsystem (NSS)

GMSC

HLR

PSTN
MSC

VLR

VLR

MSC

BSC

BSC

BS
BS

BS
Radio Sub
System (RSS)

BS

BS
Prof. Z Ghassemlooy

www.eecs.wsu.edu/~smedidi

Cellular Network - RSS

Base Station Subsystem (BSS):
Base Transceiver Station (BTS)
including transmitter, receiver, antenna

Base Station Controller (BSC)

switching between BTSs

controlling BTSs
network resources management
mapping of radio channels (Um) onto terrestrial channels (A
interface)

BSS = BSC + BTS + interconnection

Mobile Stations (MS)

www.eecs.wsu.edu/~smedidi
Prof. Z Ghassemlooy

Cellular Network - NSS

The main component of the public mobile network
switching, mobility management, interconnection to other
networks, system control

Mobile Services Switching Center (MSC)

Connecting several BSC
Controls all connections via a separated network to/from a
mobile terminal

Home Location Register (HLR)

Central master database containing user data, permanent and
semi-permanent data of all subscribers assigned to the HLR

Visitor Location Register (VLR)

Local database for a subset of user data, including data about
all user currently in the domain of the VLR
Prof. Z Ghassemlooy

www.eecs.wsu.edu/~smedidi

Cellular Network - MSC

Its roles are:

Switching and additional functions for mobility support

network resources management
interworking functions via Gateway MSC (GMSC)
integration of several databases

Its functions are:

specific functions for paging and call forwarding

termination of SS7 (signaling system no. 7)
mobility specific signaling
location registration and forwarding of location information
provision of new services (fax, data calls)
support of short message service (SMS)
generation and forwarding of accounting and billing information
www.eecs.wsu.edu/~smedidi
Prof. Z Ghassemlooy

Cellular Network - Operation Subsystem

Enables centralized operation, management, and maintenance
of all cellular subsystems
Authentication Center (AUC)
generates user specific authentication parameters on request of a VLR
authentication parameters used for authentication of mobile terminals
and encryption of user data on the air interface within the system

Equipment Identity Register (EIR) for Mobile Identification

Number (MIN)
registers mobile stations and user rights
stolen or malfunctioning mobile stations can be locked and sometimes
even localized

Operation and Maintenance Center (OMC)

different control capabilities for the radio subsystem and the network
subsystem
www.eecs.wsu.edu/~smedidi
Prof. Z Ghassemlooy

Cellular Network - Mobile Registration

VLR

MSC

VLR

HLR

MSC

Terminal Moves
into area

Send MIN
MIN
Update
location

Cancel
location

Cancel
location

Update
location
CLR Cancel
Location Result

ULR
ULR

CLR

ULR Update
Location Result

CLR
Home Location Register (HLR)
Visitor Location Register (VLR)

www.eecs.wsu.edu/~smedidi
Prof. Z Ghassemlooy

Cellular Network - Mobile Terminated Call

1- Calling a mobile unit
2- Call forwarding to GMSC
3- Signal call setup to HLR
4&5- Request MSRN from VLR
6- Forward responsible
1
MSC to GMSC
7- Forward call to current MSC
8&9- Get current status of MU
10&11- Paging of MSU
12&13- MU answers
14&15- Security checks
16&17- Call set up connection

HLR

VLR

5
8
14

3 6

9
15

7
PSTN
2

GMSC

MSC

10

BSS
11

10

13
16

BSS
11
11

10

BSS
11

12
17

MU
www.eecs.wsu.edu/~smedidi

Prof. Z Ghassemlooy

Cellular Network - Mobile Originated Call

VLR
3

1&2- Connection request

PSTN
3&4- Security check
5-8- Check resources (free circuit)
9&10- Call set up

5
GMSC

MSC
8
2

1
MU

10

BSS

www.eecs.wsu.edu/~smedidi
Prof. Z Ghassemlooy

Cellular Network MTC and MOC

MS

MTC

MS

BTS

BTS

MOC

paging request
channel request

channel request

immediate assignment

immediate assignment

paging response

service request

authentication request

authentication request

authentication response

authentication response

ciphering command

ciphering command

ciphering complete

ciphering complete

setup

setup

call confirmed

call confirmed

assignment command

assignment command

assignment complete

assignment complete

alerting

alerting

connect

connect

connect acknowledge

connect acknowledge

data/speech exchange

data/speech exchange
Prof. Z Ghassemlooy

www.eecs.wsu.edu/~smedidi

Steps in Controlled Call between Mobile Users

Mobile unit initialization

Mobile-originated call
Paging
Call accepted
Ongoing call
Handoff

Additional functions
Call blocking
Call termination
Call drop
Calls to/from fixed and remote mobile subscriber
Prof. Z Ghassemlooy

Handoff (Handover)
The process of switching a user from one cell to
another while a conversion is in progress.
It is a complex procedure because the base stations
have to calculate exactly when a user is crossing
the cell boundary. This could take several
seconds, so if the mobile user is moving too fast
the call will be dropped.
Speed limit:
Analogue systems: 100 km/h
Digital systems: 300 km/h
Some systems can complete handoff to the cruising speed
of an airliner.
Prof. Z Ghassemlooy

Handoff - Types
No handoff
The most simple
A new call is made once a mobile unit has moved out of the range of a
base station.
Not common, since it takes up to 30 sec. to set up a new call

Hard handoff
Mobile unit need to break its connection with on BS before connecting to
another
Not too reliable to establish a new call.
A cell could be already full or no cell being available at all.
Repeated handoff in areas with poor power reception within the same cell
since no other BS can accept the call.

Results in a noticeable break in conversation especially when MU is

moving fast between small cells

Soft handoff
A new link is set up to BS in the new cell before the old one is dropped.
Reliable, calls are dropped only if MU is moving very fast.
A connection with two different BSs is rather difficult with existing systems.
3G overcomes this problem.
Prof. Z Ghassemlooy

Handoff - Types
1

2
MU
MU

MU

BTS

Inter-cell handoff: MU
moving from its current cell to
the adjacent cell using the same channel

MU

BTS

BTS

BTS

BSC

BSC

BSC

MSC

MSC

Intra-cell handoff: MU moving from its

current cell to the adjacent cell using a new channel

www.eecs.wsu.edu/~smedidi
Prof. Z Ghassemlooy

Handoff - Operation
Is based on periodical measurements of the received
signal strength and link quality recorded by the MU and
passed on to the BS
BS reports the hand-off request to BSC, MSC
In 2G systems BSC handles the handover

The BS with the highest received signal level and an ideal

channel is detected.
Identifying new BS. The system switches the call to a
stronger-frequency channel in a new site without
interrupting the call or alerting the user
Allocation of voice and control signals to channels
associated with the BS. During a call, two parties are on
one voice channel
If there is no new BS, the hand-off fails and the call is
terminated.
Prof. Z Ghassemlooy

Handoff Operation -

contd.

BS 2

MU
signal
level

BS 1

MU

Threshold a

Pr-ho

Threshold b

Prmin

Minimum

Time

Initially MU is assigned to BS1.

A call will be dropped when:
there is an excessive delay by the MSC in assigning a hand-off,
the is set too small for the hand-off time in the system.
Prof. Z Ghassemlooy

Handoff Operation -

contd.

For successful Hand-off an OPTIMUM SIGNAL LEVEL is

required at which to initiate a Hand-off.
Once a particular signal level is specified, as the minimum
useable signal for acceptable voice quality at the BS
receiver (normally at -90 dBm or -100 dBm), a slightly
stronger signal level is used as a threshold at which a
Hand-off is made. This margin is given by:

Pr handoff Pr min imum usable

If is too large, unnecessary hand-offs, which burden the MSC may
occur,
If is too small, there may be insufficient time to complete a hand-off
before a call is lost due to weak signal condition.
Prof. Z Ghassemlooy

Handoff Operation -

contd.

In deciding when to hand-off, it is important to ensure:

the drop in the measured signal level is not due to momentary
FADING
the mobile is actually moving away from the serving BS.

For this to happen the BS monitors the signal level for a

certain period of time before a hand-off is initiated.
The length of time needed to decide if a hand-off is
necessary depends:
on the speed at which the MU is moving.

If the slope of the short term average received signal level

in a given time interval is steep, the hand-off should be
made quickly.
Prof. Z Ghassemlooy

Handoff Procedure
MU

BTSold
BSCold
measurement measurement
report
result

MSC

HO decision
HO required

BSCnew

BTSnew

HO request
resource allocation
ch. activation

HO command

HO command

ch. activation ack

HO command HO request ack
HO access

Link establishment
HO complete
clear
command
clear command
clear complete clear complete
Prof. Z Ghassemlooy

HO complete

Handoff - Practical Considerations

Speed at which a MU passes through the coverage area
Cars takes seconds to pass through
Pedestrian may never need a handoff during a call

Ability to obtain new cell site:

Service providers find it very difficult to obtain new physical cell site
location in urban areas. Therefore implement what is called the
umbrella cell approach

Speed of mobile is estimated by the BS

or MSC by monitoring average signal
strength
BS may transfer high speed mobile
to the co-located microcell without MSC
intervention
Prof. Z Ghassemlooy

Large
area for
high speed
mobiles

BS
BS

BS

BS

BS
BS

For low
speed
traffic

Handoff - Practical Considerations

Cell dragging:
Mainly in micro cell systems
Results from pedestrian: In urban area, because of line
of sight radio path strong signal is received by the BS
As the mobile moves away from the BS, the average
signal strength does not decay rapidly. This creates a
few problems;
Handoff-problem: The user is well outside the desired range,
and with the signal strength within the cell still being strong,
therefore no handoff.
Interference
Management problem.
Prof. Z Ghassemlooy

Handoff Performance Metrics

Cell blocking probability probability of a new call being blocked
Call dropping probability probability that a call is terminated due
to a handoff
Call completion probability probability that an admitted call is
not dropped before it terminates
Probability of unsuccessful handoff probability that a handoff is
executed while the reception conditions are inadequate
Handoff blocking probability probability that a handoff cannot
be successfully completed
Handoff probability probability that a handoff occurs before call
termination
Rate of handoff number of handoffs per unit time
Interruption duration duration of time during a handoff in which
a mobile is not connected to either base station
Handoff delay distance the mobile moves from the point at
which the handoff should occur to the point at which it does occur
Prof. Z Ghassemlooy

Mode of Communication
Frequency Division Duplex (FDD)
Uses two different frequency bands (uplink and
downlink)
A symmetric communication channel (uplink and
downlink use the same capacity)

Prof. Z Ghassemlooy

Mobile Positioning
Mobile positioning refers to determining the
position of the mobile device. Its purpose is to
provide location-based services (LBS), including
wireless emergency services
Mobile location refers to the location estimate
derived from the mobile positioning operation.
Methods:
Network based
Handset based positioning..

Prof. Z Ghassemlooy

Mobile Positioning Network Based

Uses mobile network + network-based position determination
equipment (PDE)
SS7 and Mobile Positioning (SS7 is a communications protocol that
provides signalling and control for various network services and
capabilities.
The easiest method
MSC launch a SS7 message containing the cell of origin (COO) or cell ID
(of the corresponding cell site currently serving the user).
Covering a large area, the COO may be used by LBS to approximate the
location of the user.
A large degree of uncertainty that should be taken into account by the LBS
application in term of required quality of service (QOS).

Network based PDE

Angle of Arrival Method - between the mobile phone and the cellular
antenna.
Time of Arrival Method - of signals between the mobile phone and the
cellular antenna
Radio Propagation Techniques - utilize a previously determined mapping
of the radio frequency (RF) characteristics to determine an estimate of the
mobile device position
Hybrid Methods
Prof. Z Ghassemlooy

Mobile Positioning Handset Based

Subscriber Identity Module (SIM) Toolkit
Positioning information may be as approximate as COO or more
precise through additional means such as use of the mobile
network operation called timing advance (TA) or a procedure
called network measurement report (NMR).
SIM toolkit is a good technique to obtain position information
while the mobile device is in the idle state.

Enhanced Observed Time Difference (E-OTD)

Global Positioning System (GPS)
The most accurate (when satellites are acquired/available), but
is often enhanced by additional network equipment.

Mobile IN Technologies
Prof. Z Ghassemlooy

Cellular System - Power Control

It desirable to introduce dynamic power control
To have a high SNR:
received power must be sufficiently above the background
noise for effective communication Pr > NT (Noise total
power)
Rapid changes to the received power is due to:
- Reflection - Diffraction and - Scattering
To reduce co-channel interference, alleviate health
concerns, save battery power:
minimize mobile transmitted power
To equalize the received power level from all mobile units at
the BS
Prof. Z Ghassemlooy

Power Control - Types

Open-loop power control
Depends solely on mobile unit
No feedback from BS
Continuous transmission of a Pilot Signal, thus allowing
MU to use it for
Timing for forward link (BS MU)
Phase reference for demodulation
Power control

Assumptions: Forward and reverse links are correlated

Not as accurate as closed-loop, but can react quicker to
fluctuations in signal strength

Prof. Z Ghassemlooy

Power Control - Types

Closed-loop power control
Adjusts signal strength in reverse channel (MU BS)
based on metric of performance in the reveres channel
Received power level
Received SNR
Or received bit error rate (BER)

BS makes power adjustment decision and communicates to

a power adjustment command to the mobile on a control
channel

Prof. Z Ghassemlooy

Interference
Interference is the major limiting factor in the
performance of cellular radio systems. Sources of
interference include:

another mobile in the same cell

a call in progress in the neighbouring cell
other BS s operating in the same frequency band
any non-cellular system which inadvertently leaks energy
into the cellular frequency band.

Interference effects:
on voice channel causes crosstalk
on control channels it leads missed and blocked calls due to
errors in the digital signalling.
Prof. Z Ghassemlooy

Interference -

contd.

Interference is more severe in the urban areas, due to

the greater RF noise floor
large number of BSs and mobiles

Interference has been recognised as a major bottleneck in

increasing capacity and is often responsible for dropped calls
Types of Interference

Co-channel

Adjacent
channel
Prof. Z Ghassemlooy

Power
level

Multipath

Wireless Communication System Interference

Interference
user 1

u(t)
Transmitter

Channel M

Channel 1
Channel

Co-Channel
Interference

v(t)

user M
...

noise n(t)
Forward
Channel
Reverse
Channel
Prof. Z Ghassemlooy

r(t)

Receiver

Co-channel Interference (CCI)

Is due to frequency reuse in a given coverage area.
Unlike thermal noise, which can be overcome by
increasing the signal-to-noise ratio, CCI can not be
reduced by simply increasing the signal (carrier) power
at the transmitter.
This is because an increase in carrier transmit power
increases the interference to neighbouring co-channel
cells.
To reduce CCI, co-channel cells needs to be physically
separated by a minimum distance to provide sufficient
isolation due to propagation.
Prof. Z Ghassemlooy

Co-channel Interference -

contd.

The signal-to-interference ratio (SIR) for a mobile

receiver monitoring a forward channel is given as:

SIR

S
io

Ii

SIR ~17 -19 dB

i 1

where
io = No. of co-channel interfering cells
S = Signal power from a desired BS
Ii = interference power caused by the ith interfering cochannel cell BS.
Prof. Z Ghassemlooy

Co-channel Interference -

contd.

Average received power Pr at a distance d from the

transmitting antenna is:

d
Pr P0
d0
Or in dB

d
Pr (dBm) P0 (dBm) 10nLog
d0

where
P0 = Power received at a close-in reference point in the far field
region of the antenna at a small distance d0 from the Tx antenna.
n = Path lose exponent. 2< n <4 for urban cellular.
Prof. Z Ghassemlooy

Co-channel Interference -

contd.

Lets consider the forward link where :

Desired BS
R
BS1

Di

BS1

Interfering
BSi

Mobile unit

S R

And

I i ( Di )
Prof. Z Ghassemlooy

Co-channel Interference Assuming

transmitted power of each BS
is equal
n is the same throughout the
coverage area,

If all the interfering BSs are

equidistant from the desired BS
If this distance is equal to the
distance Dcc between the cells
Since Q = Dcc/R
Prof. Z Ghassemlooy

contd.

SIR

i0

( Di )

i 1

( Dcc / R)
SIR
i0

3N
i0

Co-channel Interference - Example

For the USA AMPS cellular system which uses FM and
30 kHz channels, a 7-cell cluster might be used there
could be up to 6 immediate interference, Assuming the
fourth power propagation law, an approximate value of
the SNI would be:
Solution:
4

S
R
SIR

4
6 I ' s 6 Dcc

since Dcc/R = (3N)1/2, then

SIR = 1.5 N2 = 1.5 (7)2 = 74
in dB SIR = 10 log (74) = 19 dB.

Compared with 13 dB for GSM

Prof. Z Ghassemlooy

Co-channel Interference
Base A

SIR(dA, Dcc ) PA (dA ) PB (Dcc dA )

from base C
-60
-70

Received Power dBm

If stations A and B are using

the same channel, the signal
power from B is co-channel
interference:

Base B

log10[(Dcc / dA ) 1] dB

received power

received power
from base B

from base A

-80
-90
-100

received power
from base C

-110
-120
-130
0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Normalized Distance from Base A

Prof. Z Ghassemlooy

2.0

Spectrum Efficiency
Defined as the traffic capacity unit (i.e. number of channel /cell)
divided by the product of bandwidth and the cell area
Is dependent on the number of radio channels per cell and the
cluster size (number of cells in a group of cells):
Cellular system capacity or spectrum efficiency can be most easily
and inexpensively increased by:
subdividing cells into smaller cells
sectorising the cells.

A reuse pattern of Ns/N , Ns is the number of sectors.

Some current and historical reuse patterns are
3/7 (North American AMPS),
6/4 (Motorola NAMPS),
3/4 (GSM).

Prof. Z Ghassemlooy

How to Reduce CCI Sectorisation

(Directional Antenna)
Use of a directional antenna instead of omnidirectional antenna:
120o or 60o sector antenna
The frequency band is further subdivided (denoted 1-1,
1-2, 1-3, etc.). This does not use up frequencies
faster (same number of channels/cell)

1
2
3

1-1
2
1-2
1-3
Sector in use

120o

CCI

Cell with 3 sectors having their own frequencies and

antennas
Prof. Z Ghassemlooy

How to Reduce CCI Sectorisation

For a 7-cell cluster, the MU will
receive signals from only 2
other cluster (instead of 6 in an
G
F

B
A
E

omnidirectional antenna)

For worst case, when mobile is

at the edge of the cell
n

R
SIR
n
Dcc ( Dcc 0.7 R ) n
Desired channel

Interfering co-channel cells @ D distance

Prof. Z Ghassemlooy

How to Reduce CCI

contd.

Sequential Transmitter
Only one transmitter is being used while all the
surrounding transmitters are switched off (i.e
transmitters are turned on in turn)

time delay

Prof. Z Ghassemlooy

Adjacent Channel Interference (ACI)

Results from signals which are adjacent in frequency to the
desired signal due to imperfect receiver filters.
It can be serious if an adjacent channel user is transmitting
in very close range to a mobile unit. This is refereed to as
the NEAR-FAR EFFECT (NFF)
NFF also occurs when a mobile close to a BS transmits on
a channel close to one being used by a weak mobile.

Can be minimised by:

careful filtering
careful channel assignments:
careful frequency allocation
sequential assigning successive channels in the frequency band to
different cells.
Prof. Z Ghassemlooy

Adjacent Channel Interference -

Power spectrum

ACI

Frequency
fc1

fc2

fc3

Prof. Z Ghassemlooy

contd.

Approaches to Cope with Increasing Capacity

Adding new channels or new frequency band
GSM uses two bands in Europe: 890-960 MHZ, and 1710 1880 MHz

Decrease cell size and at the same time reduce transmit power (to
keep CCI low)
Frequency borrowing
frequencies are taken from adjacent cells by congested cells
Increase the number of cell per cluster
Cell splitting: cells in areas of high usage can be split into smaller cells

Cell sectoring
cells are divided into a number of wedge-shaped sectors, each with
their own set of channels
Microcells (100 m 1 km in diameter)
compared to the standard cell size of 2-20 km in diameter
antennas move to buildings, hills, and lamp posts

Smart antennas

Prof. Z Ghassemlooy

Cell Splitting
Consider the number of voice circuits per given service area.
If a base station can support X number of voice circuits, then cell
splitting can be used to increase capacity

Before cell splitting

After cell splitting

As shown above a rough calculation shows a factor of 4 increase.
This is the reason for using more base stations in a given area
Prof. Z Ghassemlooy

Cell Splitting
This increase does not hold indefinitely for several reasons:
Eventually the BSs become so close together that line-of-sight conditions
prevail and path loss exponent becomes less (e.g., 2 versus 4)
Obtaining real estate for increased number of base stations is difficult
As cell sizes become smaller, number of handoffs increases; eventually speed
of handoff becomes a limiting factor

Mini cells will have their own Tx and Rx antennas

Power at the boundary
of un-split cell:

Pu Ptu R

R
Power at the boundary
of a new mini cell:

Pms Ptms ( R / 2) n

Where Ptu =transmitted power un-split cell

Ptms= transmitted power from mini cell

To maintain the same CCI performance Pu = Pms

P tms Ptu / 2 n

Prof. Z Ghassemlooy

Smart Antennas
BSs transmits the signal to the desired MU
With a maximum gain
Minimized transmitted power to other MUs.

Overcomes the delay spread and multipath fading.

Two types:
Switched-beam antenna
Cell sectrisation: where a physical
channel, such as a frequency, a
time slot, a code or combination of
them, can be reused in different
minisectors if the CCI is tolerable.
Adaptive beam-forming antenna
BS can form multiple independent narrow beams to serve the MUs
(i.e. two or more MUs which are not close to each other geometrically
can be served by different beams. Therefore, the same physical
channel can be assigned to two or more MUs in the same cell if the
CCI among them is tolerable.
Prof. Z Ghassemlooy

Signal-to-Noise Ratio (SNR)

S
SNR )Total
N IT
S is the signal power
N is the total noise power at the receiver stage.
N = Nth + Namp.
IT is the total interfering signal power = CCI +ACI
Average power of thermal noise Nth = KTB

R=1 ohm

B = Bandwidth
T= Absolute temperature in degree Kelvin
K = Boltzmanns constant = 1.38 x 10-23 W/Hz/Ko
Prof. Z Ghassemlooy

What is the goal in cellular systems?

Increase capacity while minimizing the interference

Prof. Z Ghassemlooy

Prof. Z Ghassemlooy

Gary Minnaert

Glossary

AMPS: advanced mobile phone service; another acronym for analog cellular
radio
BTS: base transceiver station; used to transmit radio frequency over the air
interface
CDMA: code division multiple access; a form of digital cellular phone service
that is a spread spectrum technology that assigns a code to all speech bits,
sends scrambled transmission of the encoded speech
DAMPS: digital advanced mobile phone service; a term for digital cellular
radio in North America.
DCSdigital cellular system
ETDMA: extended TDMA; developed to provide fifteen times the capacity
over analog systems by compressing quiet time during conversations
ESN: electronic serial number; an identity signal that is sent from the mobile to
the MSC during a brief registration transmission
FCC: Federal Communications Commission; the government agency
responsible for regulating telecommunications in the United Sates.
FCCH: frequency control channel
FDMA: frequency division multiple access; used to separate multiple
transmissions over a finite frequency allocation; refers to the method of
allocating a discrete amount of frequency bandwidth to each user
Prof. Z Ghassemlooy

Glossary

FM: frequency modulation; a modulation technique in which the carrier

frequency is shifted by an amount proportional to the value of the modulating
signal
FRA: fixed radio access
GSM: Global System for Mobile Communications; standard digital cellular
phone service in Europe and Japan; to ensure interpretability between
countries, standards address much of the network wireless infra
MS or MSU: mobile station unit; handset carried by the subscriber
MSC: mobile services switching center; a switch that provides services and
coordination between mobile users in a network and external networks
MTSO: mobile telephone switching office; the central office for the mobile
switch, which houses the field monitoring and relay stations for switching calls
from cell sites to wireline central offices (PSTN)
MTX: mobile telephone exchange
NADC: North American digital cellular (also called United States digital
cellular, or USDC); a time division multiple access (TDMA) system that
provides three to six times the capacity of AMPS
NAMPS: narrowband advanced mobile phone service; NAMPS was
introduced as an interim solution to capacity problems; NAMPS provides three
times the AMPS capacity to extend the usefulness of analog systems
Prof. Z Ghassemlooy

Glossary

PCS: personal communications service; a lower-powered, higher-frequency

competitive technology that incorporates wireline and wireless networks and
provides personalized features
PSTN: public switched telephone network; a PSTN is made of local networks,
the exchange area networks, and the long-haul network that interconnect
telephones and other communication devices on a worldwide b
RF: radio frequency; electromagnetic waves operating between 10 kHz and 3
MHz propagated without guide (wire or cable) in free space
SIM: subscriber identity module; a smartcard which is inserted into a mobile
phone to get it going
SNSE: supernode size enhanced
TDMA: time division multiple access; used to separate multiple conversation
transmissions over a finite frequency allocation of through-the-air bandwidth;
used to allocate a discrete amount of frequency ban

Prof. Z Ghassemlooy

Summary

Cell Shapes & Clusters Size

Frequency Reuse
Handoff Strategies
Interference (CCI + ACI)
How to Combat Interference
Signal-to-Noise Ratio

Prof. Z Ghassemlooy

Next Lecture

Traffic Engineering

Prof. Z Ghassemlooy